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Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> Tracy, California <br /> The average linear flow rate of ground water can be calculated from 1) lie estimated background <br /> gradient across the site (0.01 foot-per-foot; 2) the value of hydraulic ,.onductivity (7.47 x 10' <br /> foot-per-minute); and 3) the estimated porosity of 35 percent (a conser•iative value for silts and <br /> clays). <br /> ' 1 <br /> R dh= K * dh * — <br /> dl n <br />' Where: q, = Average linear pore velocity <br /> K = Hydraulic conductivity <br />' dh/dl = Ground water gradient <br /> n = Porosity <br />' Based on these values, the greatest average linear ground water velocity is approximately <br /> 11.2 feet-per-year. This velocity is equivalent to the conservative trans})ort rate of a nonreactive <br /> solute. However, considering that this velocity estimate ignores sorption, degradation, and <br /> volatilization processes, the effective rate of dissolved-phase hydrocarbon transport is expected <br /> to be lower. <br /> 1.2.7 Summary of Remediation Activities <br /> 1.2.7.1 Soil Remediation Activities <br /> Balch Petroleum, Inc. (BPI) was contracted b Mobil Oil Corporation to perform excavation of <br /> Y rP <br /> hydrocarbon-affected soil to below the capillary fringe. The areal extf=nt of the soil excavation <br /> was based on the results of in situ soil characterization conducted in April 1990. The limits of <br /> the soil excavation area are shown in Figure 1-4. A total of approximately 6,100 cubic yards <br />' of soil was excavated from the site; the excavated soil was disposed of at either Liquid Waste <br /> Management (Class H facility) or Durham Road Landfill (Class III facility), based on waste <br />' characterization. <br /> 30-0136-11 <br /> 1-10 <br />